Christopher S. Reynolds 
and Mitchell C. Begelman 
JILA, University of Colorado, Campus Box 440, Boulder, CO
80309-0440
Department of Astrophysical and Planetary Sciences,
University of Colorado, Boulder, CO 80309-0391
We suggest that extragalactic radio sources are intermittent on timescales
of 
- 
yr. Using a simple spherical model of a
cocoon/shock system, it is found that inactive sources fade rapidly in
radio luminosity but the shock in the ambient medium continues to expand
supersonically, thereby keeping the whole source structure intact during
the inactive phases. The fading of inactive sources, and the effect of the
intermittency on the expansion velocity, can readily explain the observed
over-abundance of small radio sources. In particular, the plateau in the
observed distribution of sizes found by O'Dea & Baum (1997) can be
interpreted as being due to intermittency. The model predicts that very
young sources will be particularly radio luminous, once the effects of
absorption have been accounted for. Furthermore, it predicts the existence
of a significant number of faint `coasting' sources. These might be
detectable in deep, low-frequency radio maps, or via the X-ray and optical
emission line properties of the shock front.